The Collaborative International Dictionary of English v.0.48:

Improvement \Im*prove"ment\, n.
   1. The act of improving; advancement or growth; promotion in
      desirable qualities; progress toward what is better;
      melioration; as, the improvement of the mind, of land,
      roads, etc.
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            I look upon your city as the best place of
            improvement.                          --South.
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            Exercise is the chief source of improvement in all
            our faculties.                        --Blair.
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   2. The act of making profitable use or applicaton of
      anything, or the state of being profitably employed; a
      turning to good account; practical application, as of a
      doctrine, principle, or theory, stated in a discourse. "A
      good improvement of his reason." --S. Clarke.
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            I shall make some improvement of this doctrine.
                                                  --Tillotson.
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   3. The state of being improved; betterment; advance; also,
      that which is improved; as, the new edition is an
      improvement on the old.
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            The parts of Sinon, Camilla, and some few others,
            are improvements on the Greek poet.   --Addison.
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   4. Increase; growth; progress; advance.
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            There is a design of publishing the history of
            architecture, with its several improvements and
            decays.                               --Addison.
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            Those vices which more particularly receive
            improvement by prosperity.            --South.
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   5. pl. Valuable additions or betterments, as buildings,
      clearings, drains, fences, etc., on land or premises; as,
      to convey the land together with all improvements.
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   6. (Patent Laws) A useful addition to, or modification of, a
      machine, manufacture, or composition. --Kent.
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WordNet (r) 3.0 (2006):

improvement
    n 1: a change for the better; progress in development [syn:
         improvement, betterment, advance]
    2: the act of improving something; "their improvements increased
       the value of the property"
    3: a condition superior to an earlier condition; "the new school
       represents a great improvement" [syn: improvement,
       melioration] [ant: declination, decline]